Increasingly stringent statutory provisions relating to permitted pollutant emissions from motor vehicles, in which internal combustion engines are disposed, mean that pollutant emissions have to be kept as low as possible during operation of the internal combustion engine. This can be achieved on the one hand by reducing the pollutant emissions, which result during combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. Exhaust gas post-treatment systems can also be used in internal combustion engines, to convert the pollutant emissions produced during the combustion process of the air/fuel mixture in the respective cylinders to harmless substances. Exhaust gas catalytic converters are used for this purpose, converting carbon monoxide, hydrocarbons and nitrous oxides to harmless substances. Both the specific influencing of the production of pollutant emissions during combustion and the conversion of pollutant components in a highly efficient manner using an exhaust gas catalytic converter require a very precisely set air/fuel ratio in the respective cylinder.
A method for the cylinder-selective regulation of an air-fuel mixture to be burned for an internal combustion engine with a number of cylinders is known from DE 199 03 721 C1, wherein the lambda values for different cylinders or cylinder groups are sensed and regulated separately. A probe/evaluation unit is provided for this purpose, in which the exhaust gas probe signal is evaluated with time resolution, thereby determining a cylinder-selective lambda value for each cylinder in the internal combustion engine. Each cylinder is assigned an individual regulator, configured as a PI or PID regulator, the controlled variable of which is a cylinder-specific lambda value and the reference variable of which is a cylinder-specific target value for the lambda. The manipulated variable of the respective regulator then influences fuel injection in the respectively assigned cylinder.
The quality of cylinder-specific lambda regulation depends to a large degree on how precisely the measuring signal of the exhaust gas probe detected at the respective sampling time is assigned to the exhaust gas of the respective cylinder.
An internal combustion engine with a number of cylinders and injection valves assigned to the cylinders is known from EP 0 643 213 A1. An exhaust gas probe is disposed in the exhaust gas tract and its measuring signal is characteristic of the air/fuel ratio in the respective cylinders. The exhaust gas probe generates a measuring signal, which is converted to digital values by means of an A/D converter and stored in a buffer unit. To assign the respective fuel ratio to the respective cylinder, the respective buffered value to be read out is determined as a function of whether a desired air/fuel ratio is leaner than the stoichiometric air/fuel ratio. According to a further embodiment the buffered value to be read out is determined as a function of the atmospheric pressure.
It is known from US 2002/0026930 A1 that a measuring signal from an exhaust gas probe can be sampled and one of the sampled values can be selected as a function of the engine speed, engine load and a selected operating mode, which can for example be a stoichiometric mode, a pre-mix combustion mode and a layer mode.